The rapid growth of the Internet and the resulting possibility of copying and dissemination of digital data have led to growing demand for ways to prevent these illegal activities and to protect copyright holders against the problems associated with pirated copies. For example, unauthorized copies may be made of an authorized distribution copy (e.g., a commercial copy) of digital data. Watermarks can provide the copyright holder with evidence that a given document of the digital data is the copyright holder's own intellectual property by comparing the document with an unauthorized or illegally disseminated copy. To discourage, prevent, or inhibit the proliferation of unauthorized copies, it is often useful to know the source of an unauthorized copy by determining the particular distribution copy that was used to make the unauthorized copy. However, it can be difficult to use watermarking to identify the distribution copy that was used to generate the unauthorized copy.
Digital watermarking can be used to determine the source of an unauthorized or illegally disseminated copy. For example, when a document is to be secured using digital watermarking, an identifier that identifies the customer who is to receive the electronic distribution copy of the document can be imperceptibly embedded in the document, along with the copyright holder's watermark. If the customer disseminates copies of the distribution copy contrary to the interests of the copyright holder, the customer can be identified based on the digital watermark, which will be contained in all of the unauthorized or illegally disseminated copies. However, when many distribution copies are to be legally disseminated to many different customers, individually digital watermarking each distribution copy has typically proven to be inconvenient and time consuming.